1. Field of the Invention
The present invention relates to an optical lens for use in small imaging devices used in small, thin electronic apparatuses such as cellular phones and personal digital assistants (PDAs), and a lens unit using the same. In particular, the invention relates to an optical lens where multiple lenses are housed in a lens barrel with the optical axes thereof aligned so that the lenses are positioned relative to each other, and a lens unit using the same.
2. Description of the Related Art
As the market for mobile terminals including an imaging device expands in recent years, a small solid-state image sensor having a larger number of pixels has been mounted on such imaging devices. With the miniaturization and pixel number increase of image sensors, it has been common to form an imaging lens unit by overlapping multiple lenses in a lens barrel.
Overlapping multiple lenses to form a lens unit requires alignment of the optical axes of the overlapped lenses. Examples of known methods for such optical axis alignment include JP-A-2002-286987. JP-A-2002-286987 discloses a method of forming plastic lenses in such a manner that the outer diameters thereof are slightly smaller than the inner diameter of a lens barrel and inserting the plastic lenses into the lens barrel while maintaining clearance therebetween in the diameter direction, as well as forming an optical axis-centered, tapered conical surface in the overlap surface of each lens and pressing the last-stage plastic lens, which is freely fit into the lens barrel, in the optical axis direction so as to fit the conical surfaces of the lenses with each other so that the last-stage lens is bonded and fixed to the lens barrel with the optical axes of the lenses aligned.
Meanwhile, when overlapping multiple lenses, part of light incident on each lens may be reflected internally within the lens, becoming unwanted light which is not conducive to image formation (hereafter simply referred to as “unwanted light”). This unwanted light may be reflected on the image sensor, causing ghosting and flare. Examples of known methods for reducing such unwanted light to reduce ghosting and flare include one disclosed in JP-A-2010-32902, which includes disposing an annular shading plate outside the effective areas of the lens portions of the lenses and interposing the shading plate between the lenses when inserting the lenses into a lens barrel. Specifically, as shown in FIG. 3, two lenses, R1 and R2, are overlapped with a shading plate L interposed therebetween. Thus, the optical path of unwanted light H reflected by a concave surface R1a of a second surface of the lens R1 is blocked by the shading plate L. This prevents the reflection of the unwanted light H on the effective area of the image sensor.